ARM: dts: NSP: Add I2C support to the DT

Add I2C support to the Broadcom Northstar Plus Device Tree.  Since no
driver changes are needed to enable this hardware, only the device tree
changes are required to make this functional.

Signed-off-by: Jon Mason <jonmason@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>